Nitrofurantoin is metabolised in the liver and most body tissues while about 30 to 40% of a dose is excreted rapidly in the urine as unchanged nitrofurantoin. Some tubular reabsorption may occur in acid urine. Average doses give a concentration of 50 to 200 micrograms/mL in the urine in patients with normal renal function.
Martindale Complete Drug Reference. Pharmaceutical Press, (via medicines Complete), last reviewed 20/8/2010

Nitrofurantoin 50mg was administered in a three-way random crossover design to six healthy men. After a 45-mM intravenous infusion the plasma concentration data could be described by a two-compartment open-body model with a terminal halflife of 58.1 ± 15 min. Although absorption appeared to be complete, the absorption rate profile was complex and erratic. Two subjects failed to achieve the minimum effective urine concentration of 32 microgramslml. After the intravenous infusion 47 % of the dose was excreted unchanged in the urine and 1.2 % was recovered as the reduced metabolite aminofurantoin. After oral administration, 34% was excreted unchanged in urine.
Nitrofurantoin disposition. Hoener B, Patterson SE. Clin Pharmacol Ther. 1981 Jun;29(6):808-16.